1. Field of the Invention
The present invention relates to a metal halide lamp free from Hg, a headlight apparatus for vehicles using the particular metal halide lamp, and a method of manufacturing the metal halide lamp free from Hg.
2. Description of the Related Art
A metal halide lamp is a lamp including a light emitting tube having various metal halides sealed therein together with mercury and a rare gas for improving the light emitting efficiency and the color rendering properties of the lamp. The metal halide lamp, which is featured in its high efficiency and high color rendering properties, is widely used for general illumination such as shop illumination and road illumination. Further, a metal halide lamp has come to be used as a light source in the headlights for a vehicle.
As described above, mercury is generally used as a part of the discharge medium included in the conventional metal halide lamp. However, environmental problems have become serious nowadays, with the result that it is required in the field of illumination to decrease the amount of use of mercury giving a heavy environmental burden. Further, it is important to eliminate mercury completely from the lamp. Such being the situation, several measures have been proposed to date for eliminating mercury from the metal halide lamp.
For example, proposed in Japanese Patent Disclosure (Kokai) No. 11-238488 is a metal halide lamp having a first metal halide, i.e., a halide of the main light emitting substance such as scandium (Sc), sodium (Na) or a rare earth element, and a second halogenated metal, i.e., a halide of, for example, zinc (An), manganese (Mn), aluminum (Al) or gallium (Ga) having a high vapor pressure, sealed therein together with a rare gas. Also, disclosed in Japanese Patent Disclosure No. 11-307048 is a metal halide lamp further having a third metal halide, i.e., a halide of, for example, yttrium (Y) or indium (In), sealed therein. Each of these prior arts is intended to deal with various problems arising from the non-use of mercury.
In the metal halide lamp that does not use Hg, i.e., an Hg-less metal halide lamp, new problems arise as described below based on the situation that Hg is not used as a part of the discharge medium. Specifically, in the Hg-less metal halide lamp, the cloudiness phenomenon or the blackening phenomenon tends to be brought about at an earlier stage, compared with the conventional metal halide lamp having Hg sealed therein, with the result that it is difficult to obtain a practically sufficient lamp life in the Hg-less metal halide lamp. To be more specific, the lamp life obtained in the Hg-less metal halide lamp is about 500 hours, compared with about 3000 hours for the conventional metal halide lamp having Hg sealed therein.
It is known in the art that the life of the discharge lamp is affected by the impurities such as water present in the lamp. For example, disclosed in Japanese Patent Disclosure No. 11-329350 is a high pressure discharge lamp or a discharge lamp such as a metal halide lamp, in which the amount of the OH groups contained in the quartz glass constituting the discharge container is lowered to, for example, 10 ppm or less and, at the same time, a rare gas having hydrogen, oxygen and the compound thereof (H2O) removed therefrom, e.g., a rare gas having the water content lowered to 5 ppm or less in the molar ratio, is sealed in the discharge container. Further, disclosed in Japanese Patent Disclosure No. 2001-357818 is a metal halide lamp having Hg sealed therein, in which the water content of the gas sealed in the discharge space is lowered to 130 ppm or less.
However, in the Hg-less metal halide lamp, it is impossible to prolong sufficiently the life of the lamp by simply decreasing the amount of the OH groups contained in the quartz glass and by decreasing the amount of water contained in the rare gas, with the result that the Hg-less metal halide lamp having a long life cannot be obtained with high reproducibility. It is considered reasonable to understand that the decrease of the lamp life of the Hg-less metal halide lamp is affected by the water contained in the metal halide sealed in the discharge container as a light-emitting material.
To be more specific, a light-emitting material containing a metal halide having a high vapor pressure, i.e., the halide of Zn, Mn or Al referred to above, is used in the Hg-less metal halide lamp, so as to permit the light-emitting material to ensure the required lamp voltage in place of mercury. In the Hg-less metal halide lamp of this type, the amount of the metal halide sealed in the lamp is markedly larger than that sealed in the metal halide lamp having Hg sealed therein. It follows that, even if the metal halide raw materials used as the materials for manufacturing the metal halide lamps are equal to each other in water content, it is considered reasonable to understand that the amount of water released from the metal halide into the discharge space for the Hg-less metal halide lamp is larger than that for the metal halide lamp having Hg sealed therein.
Further, where a required lamp voltage is ensured by using a metal halide having a high vapor pressure, the amount of the halogen gas such as a free iodine gas is increased with increase in the concentration of the metal halide caused by the evaporation of the metal halide during lighting. It is considered reasonable to understand that, if the halogen gas such as a free iodine gas is present in a large amount, erosion and deposition of SiO2 (migration phenomenon of Si) are caused to take place in the quartz glass constituting the discharge container so as to bring about the cloudiness phenomenon (blackening phenomenon). Incidentally, in the metal halide lamp having Hg sealed therein, HgI2, for example, is formed so as to suppress the generation of halogen gas such as free iodine gas.
On the other hand, measures for decreasing, for example, the water content and the oxygen content are taken in respect of the metal halide itself used as the light-emitting material of the metal halide lamp, as disclosed in, for example, Japanese Patent Open Publication (Toku-hyo) No. 2000-516901. To be more specific, disclosed in this prior art are metal halide particles and metal halide pellets having the oxygen content lowered to 200 ppm or less and having the hydrogen content lowered to 100 ppm or less. These metal halide particles or pellets are manufactured by using a manufacturing apparatus which permits preventing an increase in the water content, under an environment that, for example, the dew point is not higher than −50° C.
However, even in the case where the particles or pellets of the metal halide referred to above are used as a light-emitting material of the Hg-less metal halide lamp, it is impossible to improve the lamp life with a high reproducibility without fail. It should be noted in this connection that the moisture absorption tends to take place not only in the manufacturing process of the Hg-less metal halide lamp but also in the after-treating process and the transfer process of the manufactured Hg-less metal halide lamp so as to increase the water content. This tendency is particularly high when it comes to a light-emitting material containing a metal halide having high deliquescent properties such as HgI2, with the result that it is highly possible for the water content to be increased at the stage of sealing the light-emitting material in the metal halide lamp. It follows that the Hg-less metal halide lamp manufactured by using such a light-emitting material tends to have a short life.
As described above, in the Hg-less metal halide lamp, the decrease in the lamp life caused by the cloudiness phenomenon or the blackening phenomenon remains to be a serious problem to be solved. It is considered reasonable to understand that the decrease of the lamp life is affected by the water content in the discharge space during lighting. However, it is impossible to improve the lamp life of the Hg-less metal halide lamp with a high reproducibility by simply decreasing the amount of the OH groups in the quartz glass and by decreasing the water content in the rare gas as in the prior art. Such being the situation, it is of high importance to overcome the difficulty inherent in the Hg-less metal halide lamp so as to improve the life of the Hg-less metal halide lamp by suppressing an increase in the water content during lighting of the lamp.